A solid diffusion source comprising glass ceramic or polycrystalline particles is disclosed in the Rapp U.S. Pat. No. 4,141,738. In U.S. Pat. No. 4,033,790 there is disclosed solid diffusion sources made by hot pressing La.sub.2 O.sub.3 /P.sub.2 O.sub.5, Ce.sub.2 O.sub.3 /P.sub.2 O.sub.5 and Y.sub.2 O.sub.3 /P.sub.2 O.sub.5 compounds. The above U.S. patents are incorporated by reference.
It is desirable to provide a solid diffusion source that can be easily prepared without hot pressing, the diffusion source being made from a composition that evolves P.sub.2 O.sub.5 at a relatively low temperature at a relatively high rate.